Large diameter pipe can be manufactured by numerous processes which involve bending of the metal pipe to bring the edge of the plates together and enable welding to form a seam. Among these processes are the UOE process, the three-roll bending process, the spiral bending process and the pipe die pressing process. In pipe die pressing, one generally differentiates between a progressive folding process and the progressive die-shaping process.
In the production of pipe and especially large diameter pipe by the progressive die-shaping process, in a succession of steps, the metal plate, e.g. a steel plate, is progressively bent. The metal plate is generally prebent in a first step at its longitudinal edges in a process which is usually called crimping and which is intended to enable the longitudinal edges to ultimately be brought into butting relationship without the formation of a flat at the seam, which is to be formed by welding the butting edges together. This prebending is generally carried out in a separate edge-bending press.
The prebent plate is then subjected to progressive bending in a pipe die shaping press. The die-shaping press is comprised of a movable upper part and a stationary lower part. The stationary lower part can be a bed on which are provided a pair of bars which extend linearly parallel to one another along the press bed while a ram may be movable with the press head by the hydraulics coupled therewith to drive the upper bending die against the metal plates supported by the two bars. The ram of the press head may be a vertically disposed plate-like member which can be referred to as a bending sword and which may have the upper bending die affixed to it at its foot.
The spacing of the bars forming the lower shaping die may be variable to obtain different bend radii of curvature.
The prebent plate is generally slid into the pipe bending press and by driving of the ram against the plate, a bending force can be applied to the plate which produces a further deformation thereof. The process is repeated while gradually rotating the plate until a slit pipe or tube is formed, i.e. the edges of the pipe are brought together sufficiently that the gap between them corresponds only to the thickness of the bending sword forming the ram.
The time required for so bending the plate depends upon the number of strokes required by the press to progressively bend the plate inwardly and from the bending which can be accomplished during each stroke. The arcuate extent of the bend induced by each stroke is given by the width of the upper shaping die.
The upper shaping die which is mounted at the foot of the bending sword will normally have a width that is a multiple of the cross section or thickness of the bending sword and is a function of the radius of curvature to which the plate must be bent at each bending increment and is therefore a function of the inner radius of the pipe to be made.
The rounded upper bending die tends to engage the bending plate asymmetrically at last until the plate has been almost fully bent into a round configuration and thus the forces on the bending die are likewise asymmetrical at least for most of the bending steps. As a result bending moments are applied to the bending sword. To prevent the bending sword from being itself deformed, the bending sword of conventional presses must have greater cross sections than might otherwise be preferred and thus the slit which must be left in the rounded tube may have to be of considerable width.